This invention relates to an electrophotographic copying method and more particularly to a variable-magnification electrophotographic method.
In a conventional electrophotographic copying machine having a slit exposure optical system capable of performing variable magnification of copy size, the relative movement speed of an original and an optical system for image scanning of the original is changed to 1/m in accordance with the magnification m of a copy to be made from the original and, at the same time, the optical position of an optical system for forming the image of the original on a photoconductor is also changed in accordance with the magnification m, and a light image of the original is projected on the photoconductor which is moved at a predetermined speed so that a magnified latent electrostatic image is formed on the photoconductor.
In this type of variable magnification copying apparatus, since a unit magnified copy, an enlarged copy and a reduced copy can be made from the same original, it is possible to obtain copies with a predetermined size from originals with different sizes. However, smearing of the background and lowering of image density of the enlarged copies or reduced copies tend to become more conspicuous in comparison with the unit magnified copies. This tendency is caused by a change of the surface potential of the photoconductor since the exposure light quantity of the light image of the original to be projected on the photoconductor changes, depending upon the magnification or reduction of the copy size for the original.
More specifically, in the slit exposure apparatus, the illuminance E(m) of a light image on the photoconductor can be represented by the following equation (1) when the magnification is m: ##EQU1## wherein C.sub.1 is a constant which is determined by the brightness of a light source, illumination efficiency, F/No. of an exposure lens, the number of reflectors and the reflection efficiency of each reflector and the kind of original to be copied.
Furthermore, when the exposure time is T(m) at the magnification m, the illuminance A(m) on the photoconductor is represented by the following equation (2). EQU A(m)=E(m).multidot.T(m) (2)
Note that when the slit width of the exposed light image is set at a predetermined value on the photoconductor side, the exposure time T(m) does not depend upon the magnification m and therefore the exposure time T(m) can be substituted by a constant C.sub.2 which is determined by the ratio of the slit which formed on the photoconductor to the relative scanning speed of the photoconductor.
Hence, the equation (2) can be represented by the following equation (3): EQU T(m)=C.sub.2 ( 3)
Substituting the equation (3) and the equation (1) in the equation (2), the illuminance A(m) on the photoconductor is represented by the following equation (4): ##EQU2## FIG. 1 shows the illuminance A(m) of the photoconductor. In FIG. 1, the numbers on the ordinate indicate the relative exposure light quantity when the exposure light quantity at the unit magnification is 1.0, and the numbers on the abscissa indicate the magnification m.
As can be seen from FIG. 1, the exposure light quantity on the photoconductor increases when the original image is reduced, and decreases when the original image is enlarged.
Referring to FIG. 2, there is shown a relationship between the exposure light quantity on a photoconductor and the surface potential of the photoconductor, which indicates that as the exposure light quantity increases, the surface potential of the photoconductor decreases (refer to Electrophotography by R. M. Schaffert, M.A., Ph.D.).
Therefore, when the enlarged image or reduced image of an original is developed under the same condition as that for the unit magnification copying, particularly in the case of the reduced image, its image density becomes lower than that of the unit magnification copy, since the exposure light quantity on the photoconductor is higher than that in the case of the unit magnification copy. On the other hand, in the case of enlarged image, the smearing of the background becomes more conspicuous in comparison with that in the unit magnification copying, since the surface potential of the photoconductor is higher than that in the case of unit magnification copying.
In the variable magnification copying machine, even if a variable magnification function is simply added to the conventional unit magnification copying machine, it has a shortcoming that image quality of each copy changes depending upon the variation of the copy magnification.
As the other conventional variable magnification copying machines, the following types have been proposed: In one type of copying machine, the change of image quality caused by the change of magnification is ignored. In another type of copying machine, in order to prevent change of image quality caused by the change of magnification, the exposure to the photoconductor is performed through a slit and the width of the slit is changed in accordance with the magnification so that the exposure light quantity on the photoconductor is made constant irrespective of the magnification. In a further conventional copying machine, the illuminance on the photoconductor is kept constant by changing the illuminance of the lamp used for projecting the original image, in accordance with the magnification.
However, in the method of changing the width of the slit in accordance with the magnification, the slit width is narrowed in order to eliminate the shortcoming that the exposure light quantity increases when reducing the original image, and this causes diffraction of light, and a jitter of image is caused by the relative movement of the exposure optical system and the original in the scanning of image.
In the method of changing the illuminance of the illumination lamp for the original according to the change of magnification, the color temperature of the lamp changes with the change of the illuminance of the lamp and when making copies of colored originals or color copies, the change of the color temperature changes the lightness and color tone of the image.
Furthermore in the method of changing the slit width in accordance with the magnification using a copying machine having an image scanning optical system in which it is difficult to set a constant slit, more specifically in a copying machine as shown in FIG. 3, in which an image is formed on a photoconductor 3 by an exposure optical system 2 which scans fanwise over an original 1, the scanning speed of the exposure optical system 2 over the surface of the original 1 is not constant so that if a slit is formed on the side of the original 1, the scanning mechanism of the exposure optical system 2 becomes extremely complex, and if the slit is formed on the side of the photoconductor 3 in this apparatus, the optical path of a light image with respect to the photoconductor drum 3 changes in the course of the scanning and therefore, by forming a slit 4 as shown in FIG. 4, the width of the ray of light changes from .lambda..sub.0 to .lambda..sub.1, which makes the matter worse.
Thus, the method of preventing the change of the surface potential of the photoconductor, which is caused by the change of magnification, by correcting the exposure light quantity has a shortcoming that the application of the method is limited to a special copying machine or a copying machine whose magnification range is small.